R-Value Myth


The R-value is a modern fairy tale. It's a fairy tale   that has been so touted to the North American consumer that it now has a  chiseled in-stone status. But the saddest part of this fairy tale is   that the R-value by itself is almost a worthless number.

It  is impossible to define an insulation with a single number. To do so,  we must know more. So why do we allow the R-value fairy tale to  perpetuate? We don't know. We don't know if anybody knows. What we do  know is that the R-value fairy tale obviously favors fiber insulation.

Consider  the R-value of an insulation after it has been submersed in water or as  a 20 mile-per-hour wind blows through it. In either of these scenarios,  the R-value of fiber insulations goes to zero. But those same  conditions barely affect solid insulations. That's why we believe that  R-value numbers are misleading, meaningless numbers unless we know  other characteristics.

In all probability, no one would  ever buy a piece of property knowing only one of its dimensions.  Suppose someone offered a property for $10,000 dollars and told you it  was a seven. You would instantly wonder what that number referred to:  Seven acres? Seven square feet? Seven miles square? What? You would  also want to know the property's location: In a swamp? On a mountain?  In downtown? In other words, one number cannot accurately  describe anything, and that includes the value of an insulation.

Nevertheless  we have Code bodies mandating R-values of 20s or 30s or 40s. But a  fiber insulation with an R-value of 25 placed in an improperly sealed  house will allow wind to blow through it as if there were no  insulation. Maybe the R-value is accurate when the material is lab  tested. But a lab environment may not even remotely duplicate  conditions in the real world.

Consequently, we must  start asking for some additional dimensions to our insulation. We need  to know its resistance to air penetration, to free water, and to vapor  drive. We must begin demanding the R-value of an insulating material  after it is subjected to real world conditions.

As it is  currently used, an R-value is a number that is supposed to indicate a  material's ability to resist heat loss. It is derived by taking the R-value of a product and dividing it into the number one. The R-value  is the actual measurement of heat transferred through a specific  material.

Test to Determine the R-Value

The test used to  produce the R-value is an ASTM (American Society for Testing and  Materials) test. This ASTM test was designed by a committee to give us  measurement values that -- they hoped -- would be meaningful.  Unfortunately, the test was designed with a flaw or bias. Because of  the way it's designed, the test favors fiber insulations: fiberglass,  rock wool and cellulose fiber. Very little input went into the test for  solid insulations, such as foam glass, cork, expanded polystyrene or  urethane foam.

Nor does the test account for air  movement (wind) or any amount of moisture (water vapor). In other  words, the test used to create the R-value is a test in non-real-world  conditions. For instance, fiberglass is generally assigned an R-value  of approximately 3.5. It will only achieve that R-value if tested in an  absolute zero wind and zero moisture environment. Zero wind and zero moisture are not real-world. Our houses leak air, all our buildings leak air, and they often leak  water. Water vapor from the atmosphere, showers, cooking, breathing,  etc. constantly moves back and forth through walls and ceilings. If an  attic is not properly ventilated, water vapor from inside a house will  very quickly semi-saturate the insulation above the ceilings. Even  small amounts of moisture will cause a dramatic drop in a fiber  insulation's R-value — as much as 50 percent or more.

Vapor Barriers

We  are told, with very good reason, that insulation should have a vapor  barrier on the warm side. Which is the warm side of the wall of a  house? Obviously, it changes from summer to winter — even from day to  night. In a wintry 30 C (20 F) below zero environment, the inside of an  occupied house will certainly be the warm side. But during sun-shiny  summer months, the outside will be the warm side.

Sometimes a novice owner or builder will put vapor  barriers on both sides of the insulation. Vapor barriers so placed  generally prove to be disastrous. It seems the vapor barriers stop most of the moisture but not all. Consequently, small amounts of moisture  move into the fiber insulation, between the two vapor barriers and  become trapped. The moisture accumulates as the temperature swings back and forth. This accumulation can become a huge problem. It can  eventually total buckets of water that saturate the fiberglass. We have re-insulated a number of potato storages that originally were insulated   with fiberglass and a vapor barrier on both sides. Fiber insulation  needs ventilation on one side; therefore, the vapor barrier should go  on the side where it will do the most good.

Convection Losses In Loose-Fill Insulation

Most  people know that air penetrates the walls of a house. In fact, when the  wind blows across some homes, its occupants can feel it. But what most  people, including many engineers, do not realize is that there are very  serious convection currents that occur within fiber insulations. These  convection currents rotate vast amounts of air, but they are not fast  enough to feel or even measure, with any but the most sensitive  instruments. Nevertheless, the air constantly carries heat from the  underside of the fiber pile to the top side, letting it escape. If we  seal off the air movement, we generally seal in water vapor. That  additional water often condenses and can become a moisture-source that  rots the structure. The water, as a vapor or condensation, seriously  decreases an insulation value — the R-value. The only way to deal with  a fiber insulation is to ventilate. But ventilating means moving air  that also decreases the R-value.

Air Penetration

The  filter medium for most furnace filters is fiberglass — the same spun  fiberglass used as insulation. Fiberglass is used for an air filter  because it has less impedance to the air flow, and it is cheap. In  other words, air flows through a furnace filter very readily. All well  and good for a furnace filter -- but can that same material effectively  insulate a structure? Can you imagine insulating a house by stuffing  furnace filters into the walls and ceiling? Tremendous air currents  blow through the walls of a typical home. To demonstrate, hold a lit  candle near an electrical outlet on an outside wall when the wind is  blowing. That flame will flicker and may even go out. The average home  with all its doors and windows closed has a combination of air leaks  equal to the size of an open door. Even if we do a perfect job of  installing fiber insulation in our house and bring the air infiltration  close to zero from one side of the wall to the other, we still do not  stop air from moving vertically through the insulation itself, in  ceilings and walls.

Solid Insulations

The  best known solid insulation is expanded polystyrene. Other solid  insulations include cork, foam glass and polyisocyanate or  polyisocyanurate board stock. The last two are variations of urethane  foam. Each of these insulations is ideally suited for many uses. Foam  glass has been used for years on hot and cold tanks, especially in  places where vapor drive is a problem. Cork is of course a very old  standby, often used in freezer applications. EPS or expanded  polystyrene is seemingly used everywhere -- from throw away drinking  cups and food containers to perimeter foundation insulation, masonry  insulations, etc. Urethane board stock is becoming the standard for  roof insulation, especially for hot mopped roofs. It is also widely  used for exterior sheathing on many new houses. The R-value of the  urethane board stock is of course better than any of the other solid  insulations. All of these solid insulations perform far better than  fiber insulations whenever there is wind or moisture involved.

Most  solid insulations are installed as sheets or board stock, and most  suffer from one very common problem. They generally don't fit tight  enough to prevent air infiltration. And if the wind gets behind them,  it matters not how thick these board stocks are. We see this often in  masonry construction where board stock is used between a brick and a  block wall. Unless the board stock is actually physically glued to the  block wall, air will infiltrate behind it. When this happens, the board  stock becomes virtually worthless, since the air flows through the weep  holes in the brick and around the insulation negating its  effectiveness. Great care must be exercised in placing solid  insulations. The brick ties need to be fitted at the joints and then  sealed to prevent air flow behind the insulation.

Spray-in-place  polyurethane is the only commonly used solid insulation that absolutely  protects itself from air infiltration. When it is properly placed  between two studs or against a concrete block wall or wherever, the  bonding of the spray plus the expansion of the material in place  creates a total seal. It's almost impossible to overestimate this total  seal. In my opinion, most of the heat loss in the walls of a home has  to do with the seal, rather than the insulation.

Heat  does not conduct horizontally nearly as well as it does vertically.  Therefore, if a home had no insulation in its walls, but did have an  absolute airtight seal, there would not necessarily be a huge  difference in heat loss. But this would not be the case if ceiling  insulation was missing.

Spray-in-place polyurethane can  most effectively stop air infiltration. It is the only material that  properly applied fills in the corners, cripples, double studs, bottom  plates, top plates, etc. The R-value of a material is of no interest or  consequence if air can get past it.

One and a quarter inch of polyurethane sprayed properly in the wall of  a house will prevent more heat loss than all the fiber insulation that  can be crammed in the walls — even up to an eight-inch thickness. Not  only does the polyurethane provide better insulation, it provides the  house with significant addit

ional strength.